Well, when I flew A340s we had different setups. There were aircraft (A343) equipped with O2 generators, and actually there were different types installed: One was burning for 15min, the other ones for 22 min, if I recall correctly. Then there was the A346, which had a stationary oxygen system, and that was also boosted on some aircraft by adding bottles to be able to fly L888 (Almaty <-> Chengdu).
So first you need to describe the system your A340's got, and if it is bottles, you have the pressure indicated on the SD.
But the cockpit O2 should always last longer than the Pax O2, as they can survive in a dazed state of mind for some time, but concentration will be impaired in the cockpit at, say 14000' .
Nic